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Medium Voltage Circuit Breaker Course Chapter 3.0 Student .

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Medium Voltage Circuit Breaker CourseChapter 3.0Student Manual3.0Circuit Breaker Electrical Components and OperationBREAKER ELECTRICAL OPERATIONLearning ObjectivesAs a result of this lesson you will be able to:1. Recognize and understand the function of electrical components used in thecircuit breaker control circuit2. Describe the function of the anti pump circuit3. Understand the operation of the auxiliary switch “a” and “b” contacts4. Understand the function of the protective relays and how they interface with themedium voltage circuit breaker to automatically trip (open) in the event ofabnormal condition5. Name the two common protection relays used on medium voltage breakers3.1BASIC ELECTRICAL OPERATIONMedium voltage circuit breakers (air-magnetic, SF6, or vacuum type) are designedto operate electrically. This is accomplished by electrical components on the circuitbreaker operating mechanism. These electrical components operate cams, wheels,rollers, and latches to charge, close and open the breaker.Electric operation: 3.2The secondary disconnect provides power to the breaker electricalcomponents after the breaker is racked into the connect position in theswitchboard cubicle.The charging spring normally charges automatically when the breaker is inthe switchboard and the control fuses are installed.The breaker can now be closed and opened from a local control switch onthe breaker cubicle door (Figure 3-1) or from a remote location (ControlRoom).ELECTRICAL COMPONENTSEach breaker manufacturer operating mechanism design is different, but theelectrical components required to operate the breaker all have similarities.Therefore, basic schematics for the control circuit for each manufacturer generallyare similar as well.USNRC3-1Rev 0

Medium Voltage Circuit Breaker CourseChapter 3.0Student ManualCircuit Breaker Electrical Components and OperationCharging motors, close coils, trip coils and associated switches are required tooperate the breaker and the operating sequences are essentially the same foreach manufacturer. Basic schematics for the control circuit for eachmanufacturer generally is the same regardless of the breaker size and thecharge, close, and trip control pins on the secondary will nearly always be thesame for all plant breakers (i.e. if a DHP breaker charges on pin 5 and 6 allDHP’s). The auxiliary switch arrangements can be different and should alwaysbe verified prior to swapping cubicles.The following components are usually part of all stored energy breaker electricalcircuits:3.2.1 Secondary Disconnect: normally a plastic insulated block with silverplated copper fingers (male) or plates, which connect to a switchboard, ora mounted insulated block with silver-plated copper female receptaclepins or flat plates.3.2.2 Charging Motor: an electrical motor similar to a hand drill, which movesthe closing spring to a primed position (Figure 3-2).3.2.3 Close Coil: operates the close latch.3.2.4 Trip Coil: operates the trip latch.3.2.5 Y Relay/ Anti Pump coil: locks out the control circuit if the close operationis not completed.3.2.6 Auxiliary Switch (aux switch): a set of closed contacts and a set of opencontacts which are connected to the main operating shaft by a link andchanges position with the operation of the breaker main contacts. Theaux switch contacts, which are open when the breaker is open, are called“a contacts” and the contacts that are closed when the breaker is openare called “b contacts”. The “a contacts” are in the same position as thebreaker main contacts.3.2.7 Position Operated Switches: interlock the breaker to allow it to operateonly in the proper location, or change position and cut off a device. USNRCMotor Cut Off Switch: normally operated from a timing cam, whichoperates the switch to open the normally closed contacts and deenergizing the motor when the breaker is charged.It can also have a set of normally open contacts, which close andcomplete the circuit to the close

Student Manual Circuit Breaker Electrical Components and Operation USNRC 3-4 Rev 0 When the close coil is energized it will disengage the closing prop from the closing spring allowing the spring energy to move the contacts and close the breaker. Y Relay (Anti Pump): The